In web coating arrangements, in which the web is pulled upwards through a nip formed by a blade or the like and a back-up member, the spreading of coating substance onto the web usually takes part below the nip by means of a spreading apparatus. In a known arrangement the spreading apparatus comprises two rolls forming a nip through which the web is pulled. The coating substance is supplied onto at least one of these rolls, the applicator roll, wherefrom it is transferred to and spread onto the web. At high web speeds, one difficulty is due to web slip in the spreading nip because the applicator roll moves slower than the web. This causes uneven coating substance transfer to the web. Another known coater construction comprises spreading nozzles in lieu of applicator rolls. However, with this arrangement it is difficult to spread the coating substance uniformly on the web. A common feature of both these known constructions is that, in the spreading phase, relatively great amounts of surplus coating substance are removed from the web and from the spreading unit. This excess amount has to be pumped back to the spreading point, and this forms an extra, uneconomical step of the process. Further, another recycling circuit has to be provided for recycling the coating substance scraped off by the evening blade or blades. This complicates the construction and increases the manufacturing costs.
In coating units provided with a non-pressure coating applicator, that is, a coating substance applicator under atmospheric pressure, a phenomenon called barring causes problems in the form of coating layer variations in the longitudinal direction of the web, especially at high coating speeds (over 500 m/min). Exactly why barring occurs has not been completely clarified. An improved embodiment of the invention is based on the assumption that one reason for the occurrence of barring is due to non-uniform thickness of the coating layer on the web before the smoothing nip. This results in movements in the smoothing blade, and an uneven coating is produced because of these movements.
In an extrusion applicator, in which the coating substance is applied on the web under pressure, the situation is different. In applicators of this type, a fairly long smoothing gap is used in connection with the applicator for equalizing the coating substance pressure. The invention is not related to such coating methods.